The present invention relates to the field of percussive tools used in drilling. More particularly, the invention relates to the field of down-hole hammers which are actuated by the pressure of the drilling fluid, i.e. drilling mud. Generally, these tools are known in the petroleum drilling industry simply as "down-hole mud hammers".
Typically, down-hole hammers are used to effect periodic mechanical impacts upon a drill bit. Through this percussion, the drill string is able to more effectively apply drilling power to the formation, thus aiding penetration into the formation.
Previous attempts to provide hammering motion in a drill string using power sources other than drilling mud have included use of high pressure air (see, for example U.S. Pat. Nos. 4,446,929, 4,084,646, 4,054,180); electricity (see, for example U.S. Pat. No. 3,343,611); and spontaneously combusting fuels (see, for example U.S. Pat. No. 4,583,601). However, the use of such power sources either restricts the useful application of the tool to situations where cuttings may be removed by air or other gases; or requires separate piping or cabling be added to the drill string to conduct the alternative power sources to the hammer.
To counteract the first of these limitations, some have proposed using air to produce the hammering motion and then using mud or another fluid to flush the hole (or vice versa, in the case of European Pat. #0,233,038). However, this approach still requires separate piping. In addition, it requires that special seals and exclusionary devices be employed to separate the air chambers from the mud passageways. Such complexities add an increased failure potential and undesirable expense to the drilling process without achieving substantial gains in drilling rate.
Generally, mud actuated down-hole hammers convert a portion of the power resident in common drilling fluid into the periodic mechanical impacts upon the drill bit. Among the many attempts to increase down-hole drilling effectiveness, a successful down-hole mud hammer would provide particular advantages.
First, the mud hammer is able to complement other forms of drilling, such as conventional rotary practice. Hence, its effects on the overall rate of drilling are additive to those of existing technologies, rather than superceding.
Second, as its name implies, the mud hammer is able to operate on mud, i.e. common drilling fluid, which is used to flush cuttings out of the hole. Drilling mud is currently used for the majority of drilling situations. Consequently, the equipment and procedures are already in place for the supply and circulation of drilling mud. Thus, the mud hammer is compatible with existing drilling practice and requires essentially no adaptation for its use on a conventional drilling rig.
For the above reasons, the use of mud to actuate a down-hole hammer is advantageous. However, using drilling mud as an actuating fluid does not come without problems. Although several attempts have been made to achieve mud actuation of a hammer element, none of these designs are known to have been commercially successful. The problems with these designs are believed to be related to two of the physical properties of drilling mud, namely its non-compressibility, and its high abrasiveness.
Drilling mud is an incompressible fluid. Consequently, drilling mud is subject to severe pressure transients if its flow is stopped suddenly. Many prior mud hammer designs utilized the inertia of the moving column of fluid in the drill string to drive the hammer element towards the bit (see, for example Wolski U.S. Pat. No. 699,273; Zublin U.S. Pat. No. 1,861,042; Bassinger U.S. Pat. Nos. 2,764,130, 2,758,817, and 2,756,723). These designs were thus hampered by the extreme dynamic pressure loading caused by the "water hammer" effect, i.e. the condition wherein high pressure transients are created as a result of the sudden halting of a quantity of water. This condition is particularly destructive when high fluid flow rates, such as those encountered in normal oil well drilling operations, are involved. In the article "The Bassinger Rotary Percussion Drill," The Petroleum Engineer, December 1950, pp. B42-B52, Robinson Brown records that rig vibration and caving of hole walls are among the problems caused by the stopping and starting of the fluid column. Pressure transients developed in this way are also destructive in that they increase the flow potential, and thereby the erosion potential, across valve members. These pressure transients also contribute to extreme dynamic loading of these valving components.
A few mud hammer designs have been proposed to alleviate the water hammer problems by adding a separate leakage pathway to the tool (See, U.S. Pat. No. 2,774,334; U.S. Pat. No. 3,970,152; and U.S. Pat. No. 3,491,838). Although these designs have to some extent reduced the problems of pressure transients, the complete problems related to the incompressibility of mud have not been solved.
An additional problem seen by these earlier designs for mud hammers has been caused by the high abrasiveness of drilling mud. In particular, the erosion of critical tool surfaces has been a significant factor in limiting the service life of many mud hammers. In response to this erosion problem, Harris (U.S. Pat. Nos. 4,044,844 and 3,970,152) used tungsten carbide wear surfaces in critical erosion areas in an attempt to retard erosion. However, abrasive wear still prevented the commercial success of various designs.